Molten metal stirring apparatus



April 18, 1967 T. R. KENNEDY MOLTEN METAL STIRRING APPARATUS Filed Nov. 15, 1965 INVENTOR- THEODORE 1?. KENNEDY ATTOR/VEV United States Patent Office 3,314,670 Patented Apr. 18, 1967 3,314,670 MOLTEN METAL STIRRING APPARATUS Theodore R. Kennedy, Burlington, N.J., assignor to Inductotherm Corporation, Rancocas, N.J., a corporation of New Jersey Filed Nov. 15, 1963, Ser. No. 324,048 12 Claims. (Cl. 266-34) In general, this invention relates to a new and improved method and apparatus for molten metal stirring. More particularly, it is directed to the use of polyphase stirring of molten metal in a ladle to produce unidirectional stirring of molten metal therein.

The benefits of stirring molten metal for alloying, deoxidizing, degassing and the like have been known for many years. For various reasons, it has been desirable that this stirring should take place in the teeming ladle.

Since practically all ladles are essentially a substantial steel shell with a suitable refractory lining, the application of stirring forces by electromagnetic means was discouraged by the shielding action of the steel ladle shell. Further, it was generally considered hazardous and impractical to place effective induction windings inside the ladle shell.

It has also been considered advantageous to produce the type of stirring that would be markedly unidirectional from bottom to top or vice versa of the ladle. This indicated a need for a special motor action on the molten metal as distinguished from the familiar pinch effect phenomenon found in the coreless induction furnace. Stirring of molten metals in the ladle utilizing single phase alternating current sources had produced such a pinch effect" phenomenon and thus limited the amount of stirring aciheved.

The problem involved in obtaining better molten metal stirring utilizing electromagnetic means reduced to the following requirements:

(a) The molten metal had to be stirred by means external to the substantial metal ladle shell;

(b) The stirring forces had to have a significant vertical component which was reversible;

(c) The heating of the ladle shell had to be minimized to the extent that its strength and dependability would not be significantly affected; and

(d) The solution must be economical.

Requirement (a) is answered by the application of an practical and reasonably alternating magnetic field of sufiicient strength to react inductively with the molten metal and produced forces adequate for the metallurgical requirements of mixing, surface exposure, etc.

In order to provide the significant vertical component, reversible if necessary, the present invention contemplates utilizing a polyphase electrical system where an alternating field, within a designated region, has the inherent quality of varying periodically and systematically in time and space. Polyphase electrical systems which will achieve this motor phenomenon in accordance with the principles of the present invention are two phase (or four phase) and three phase (or six phase) systems.

Requirement (c) presupposes that through a specific knowledge of induction heating principles, a combination of parameters can be found such that within the boundary of a closed metallic shell, a usable alternating magnetic field may exist. Assuming the necessity for a strong physical construction, it is taken that the ladle shell would be steel and for simplicitys sake would be considered a hollow circular cylinder. The cylinder wall has a substantial dimension in thickness and electrically forms a closed loop.

For minimum heating of the shell, a metal must be selected of high physical strength consistent with low magnetic induction and low electrical conductivity.

The requirement of practicality and economics last set forth is intertwined with other electical and mechanical requirements of the over-all system and will be discussed below.

Thus, it is the general object of this invention to provide a new and improved method of and apparatus for stirring molten metal.

Another object of this invention is the provision of new and better apparatus for stirring molten metal with minimum heating of the ladle shell and maximum utilization of polyphase power supplied to the apparatus.

Still another object of this invention is the provision of new and better molten metal stirring apparatus wherein the stirring forces have a significant vertical component which is'reversible.

A further object of this invention is the provision of a new and better method of and apparatus for molten metal stirring which is practical, economical and simple to construct.

Other objects will appear hereinafter.

For the purpose of illustrating the invention, there are shown in the drawings forms which arepresently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIGURE 1 is a cross sectional view of molten metal stirring apparatus built in accordance with the principles of the present invention.

FIGURE 2 is a schematic showing of the electrical supply for the four phase alternating current coils of FIG- URE 1.

FIGURE 3 is a phase time diagram of the four phase electrical power system of FIGURES l and 2.

FIGURE 4 is a schematic showing of a six phase power supply system similar to that shown in FIGURES 1 and 2.

FIGURE 5 is a of FIGURE 4.

FIGURE 6 is a diagrammatic showing of the flux lines of coils which are spaced too close together.

FIGURE 7 is a cross sectional view similar to the showing in FIGURE 6 in which the coils are correctly spaced about the molten metal containing ladle.

In FIGURE 1, there is shown molten metal stirring apparatus built in accordance with the principles of the present invention and generally designated by the numeral 1%. The apparatus 10 includes a ladle 12 manufactured of an outer shell 14 which is a metal of high physical strength consistent with low magnetic induction and low electrical conductivity. For example, the shell 14 could be manufactured of an austenitic stainless steel in common usage. The ladle 12. has a refractory lining 16 within the shell 14. Molten metal 17 placed within the ladle 12 is to be stirred in accordance with the principles of the present invention.

Surrounding the ladle 12 there are placed four coils 18, 29, 22 and 24- connected to a polyphase low frequency source 26. By a low frequency source is meant a source operating within the frequency range of .1 cycle to 60 cycles per second. These frequencies may be obtained in polyphase systems and in the current, voltage and power ranges suitable for practical stirring applications.

However, in designing a polyphase stirring system to meet the requirements discussed previously, there are further conditions to be met. Initially, the frequency of the alternating magnetic field must be chosen so that the depth of current penetration will be less than the radius of the molten bath to be stirred. The depth of current penetration d is defined by the Well known relationship.

phase time diagram of the power supply d (inches)=1.985 /T,uf 1

where '1' is the resistivity of the metal in microhm centimeters, u is magnetic permeability, and f is frequency.

shell 14 of where K is a ratio, d is the depth of penetration for the shell material, 1- is the mean radius of the shell, and t is the wall thickness. It has been found by tests that a value of approximately for K in Equation 2 is a reasonable and workable balance between the heat produced in the ladle shell which is considered as wasted and the stirring effects produced in the molten metal as discussed above. The value of K greater than 10 is advantageous and a value as low as 5 may be tolerable.

Thus, Equation 2 can be rewritten as follows:

(1, h t (1no es) or preferably 0.41 t fimhnches) where [L5 is the magnetic permeability of the steel of the shell.

Under the conditions of Equation 4, the energy wasted in the ladle might be half the power induced in the molten metal, but tests operation have indicated this to be practical.

The proportions of the inducing coils around the ladle likewise require proper selection. Aside from providing the proper number of turns, insulation, etc., adequate for the input conditions, it is advantageous to arrive at a means for determining the number of coils to be used together with the number of phases and the order of connection. Certain considerations would normally indicate that a large number of coils and phases is desired. However, in practice, it is more economical to use coils appropriate to two phase or three phase supply systems. It is an advantage under certain conditions to use the normal complementary four phase or six phase system so that the inducing coils as a complete unit generate no significant currents in metallic structures that might enclose the stirring assembly. It is taken here that the four phase system means that there is one magnetically reversed coil for each directly connected phase coil for a two phase supply and a six phase system has one reversed coil for each directly connected coil for a three phase supply. This is indicated in FIGURES 25.

In FIGURE 2, the connection for the coils 18, 2t), 22 and 24 are shown. Coils 18 and 22 are oppositely poled with respect to each other and are connected to the same input terminals 28 and SI) of the polyphase supply 26. Coils 2th and 24 are oppositely poled with respect to each other and are connected to common input terminals 32 and 34 of polyphase supply 26. As shown in the phase time diagram of FIGURE 3, the electrical angle of the various phases are correlated with the individual coil progression. A complete set of coils may add up to an integral multiple of 360 electrical degrees.

The distance representing 360 electrical degrees or one cycle indicates the linear phase velocity. For example, if the frequency were one cycle per second and the coils 18, Zli, 22 and 24 necessary to make up the 360 degrees were spaced over a one foot distance, then the phase velocity is one foot per second.

In FIGURE 4, there is shown a six phase system comprising coils 36, 38, 40, 42, 44 and 46 for placement about a suitable ladle (not shown) in the manner described with respect to FIGURE 1. For each coil 36, 3S and 41) there is a respective coil 42, 44 and 46 oppositely poled with respect thereto. are supplied from a suitable three phase supply 48 in the same manner as was discussed with respect to the two phase system of FIGURE 2. In FIGURE 5, the correlation of the electrical angle of the various phases with the individual coil progression has been shown in a phase time diagram.

It has been found that for producing linear motion of the molten metal 17 in the ladle 12, relatively short phase coils are most effective. Analysis of this result indicated that adjacent phase coils, short in axial length relative to their diameter, have an increased degree of mutual inductance. The magnetic field mutually shared by successive coils is a measure of the translational energy appearing as a unidirectional force on the molten metal 17. This is certainly desirable. Hence, it has been found that coils should be no longer axially than about half the diameter there-of and may decrease to lower fractions within practical limits.

If the phase coils become too short without increasing the number of electrical phases, there comes a point where the complementary or reversed phase coil reduces the net flux intercepting the molten metal to an impractical value. The net flux intercepting the molten metal exists only as shallow whorls progressing along the assembly in a systematic manner but without sufilcient coupling within the molten metal.

In FIGURE 6, there is shown the four phase system of FIGURE 1 with all similar parts indicated with prime numerals. However, the system shown in FIGURE 6 is not effective for stirring the molten metal as the coils are too short with respect to their diameter.

The spacing between oppositely polarized coils 18' and 22' shown in FIGURE 6 has caused bucking of the mutual flux between phase coils l8 and 22 so that the net flux exists only at the extreme outer edges of the molten metal 17'.

Spacing between oppositely polarized coils of the same phase should, therefore, be such that the distance between the axial centers of the said oppositely polarized coils (18 and 22 or 20' and 24'), should not be less than twice the sum of the radial distance between the inside of the coil and the metal, plus one-fifth the depth of current penetration d into the metal as per Equation 1. For assemblies using the same number of coils as the electrical phase number, the axial coil length may range between the values of one-half the coil diameter down to a value equal to the radial distance between coil and metal. In this last assembly, it is assumed that the coils are connected for consistent progression in electrical degrees but there are no oppositely polarized coils.

However, as stated previously, the preferred embodiment of the present invention utilizes oppositely polarized coils so as to prevent the generation of significant currents in the metallic structures that might enclose the stirring assembly.

In FIGURE 7, there is shown a four phase system with corrective spacing of the individual phase coils. The system 56* of FIGURE 7 includes a ladle 52 having an outer shell 5'4 and a refractory liner 56 within which is placed a molten metal bath 58.

Four phase coils 6t), 62, 64 and 66 supplied by a two phase source 68 are utilized to stir the molten metal 58. Oppositely polarized coils 69 and 64 are spaced one from another such that the distance between their axial centers 76 and 72 is not less than twice the sum of the radial distance r between the inside of the coils 60 and. 64 and the molten metal 58 plus one-fifth the depth of current penetration into the metal (d as per Equation 1. The

The coils 36-46 same spacing is also provided between oppositely polarized coils 62 and 66.

In the embodiment shown in FIGURE 7, which included the complementary or 180 degree polarized coils 62 and 66, it is assumed that space between opposite flux polarities of phase coils 60 and 64 is closely filled with similar coils 62 and 66 connected in the proper phase order so that there is a uniform flux progression from coil to coil through-out the entire electrical cycle. All coils are considered to be substantially the same axial length, but may vary diametrically to accommodate a taper of the ladle.

If necessary, the phase coils can be cooled and protected from mechanical or other damage. For special protection, as shown in FIGURE 7, a non-magnetic steel liner 74 is provided for the coil assembly which acts as a winding form for each coil. Minimum heating of this liner 74 is achieved by designing the liner in accordance with an equation similar to Equation 4 where where I is the thickness of the protective liner, 7,, is the resistivity of the liner material in microhm centimeters f is the frequency and up is the magnetic permeability of the liner material. The liner 74 may be further protected by a refractory wash or coating to protect against damage from metal spillage.

Thus, the requirements initially stated with respect to producing a better molten metal stirring apparatus has been achieved by providing a polyphase supply which eliminates extraneous magnetic fluxes and mixes the molten metal in one direction uniformly from top to bottom of a ladle in a manner which is reversible. Further heating of the ladle shell has been minimized by designing the thickness of the shell in accordance with Equations 2, 3 and 4. Further, eflicient operation of the polyphase mixing has been enhanced by setting the ratio of the depth of current penetration between .3 and .9, preferably between .5 and .8 of the radius of the molten metal, while spacing oppositely polarized coils from each other a distance not less than twice the sum of the radial distance between the inside of the coil and the molten metal plus one-fifth the depth of current penetration into the metal.

Still further, the polyphase coils can be mounted on a liner which will have minimal heating if it follows the dictates set forth in Equation 5.

Thus, molten metal mixing is achieved in an economical and practical manner by staying within the guide lines set forth above.

If desired, a three phase system with phase coils spaced may be used as a polyphase source. The 60 spacing is achieved by the utilization of a reversed middle coil. This three phase system would add up to only 180 and thus would induce an instantaneous magnetic field which might affect surrounding structures or cause heating of the metal in the ladle but would still be effective to achieve the desired stirring.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

1 claim! 1. Molten metal stirring apparatus comprising a ladle for receiving molten metal to be stirred, a plurality of phase coils wound around said ladle along the vertical axis thereof, said coils being axially aligned and spaced one from another, a low frequency polyphase supply system supplying electrical power to said coils, said coils progressively varying in electrical phase angle, said coils being connected to include one magnetically reversed coil for each coil directly connected to the polyphase supply system.

2. Molten metal stirring apparatus comprising a ladle for receiving molten metal to be stirred, a plurality of phase coils wound around said ladle along the vertical axis thereof, said coils being axially aligned and spaced one from another, said coils being external of said ladle, a low frequency polyphase supply system supplying electrical power to said coils, said coils progressively varying in electrical phase angle, said ladle including a metal, non-magnetic outer shell with a refractory liner, the thickness of said shell i being less than d /Sr where a is the depth of current penetration of the steel shell, and r is the mean radius of the shell.

3. The molten metal stirring apparatus of claim 2 wherein the shell thickness r; is equal to or less than in inches, where T is the resistivity of the steel of the shell in microhm centimeters, ,u is the magnetic permeability of the steel of the shell, and f is the frequency of the applied electric power, and r is the mean radius of the shell.

4. Molten metal stirring apparatus comprising a ladle for receiving molten metal to be stirred, a plurality of phase coils wound around said ladle along the vertical axis thereof, said coils being axially aligned and spaced one from another, said coils being external of said ladle, a low frequency polyphase supply system supplying electrical power to said coils, said coils progressively varying in electrical phase angle, said phase coils including two sets of coils, each coil in one set of coils being provided with a respective magnetically reversed coil in the second set of coils, associated coils in said first and second set being spaced axially one from another with at least one non-associated phase coil being placed between each pair of associated coils in consistent phase progression, said coils adding up to integral multiples of one cycle of the applied electric power.

5. The molten metal stirring apparatus of claim 4 wherein each of said phase coils is shorter axially than the radius of the phase coil.

6. The molten metal stirring apparatus of claim 4 wherein the distance between associated coils is equal to or greater than twice the sum of the radial distance between the inside of the coil and the molten metal in the ladle, plus one-fifth the depth of current penetration d into the molten metal, the depth of current penetration d in inches being equal to 1.985VW where 7- is the resistivity of the metal in microhm centimeters, ,u. is the magnetic permeability of the molten metal, and f is the frequency of the applied electric power.

7. Molten metal stirring apparatus comprising a ladle for receiving molten metal to be stirred, a plurality of phase coils wound around said ladle along the vertical axis thereof, said coils being axially aligned and spaced one from another, said coils being external of said ladle, a low frequency polyphase supply system supplying electrical power to said coils, said coils progressively varying in electrical phase angle, each of said phase coils having an axial coil length equal to a value between the coil radius and a value equal to the radial distance between the coil and the molten metal, the number of phase coils being equal to the phase number of the supply system.

8. Molten metal stirring apparatus comprising a ladle for receiving molten metal to be stirred, a plurality of phase coils wound around said ladle along the vertical axis thereof, said coils being axially aligned and spaced one from another, said coils being external of said ladle, a low frequency polyphase supply system supplying electrical power to said coils, said coils progressively varying in electrical phase angle, a non-magnetic liner, said phase coils being wound about said non-magnetic liner to protect said coils from damage, said liner having a thickness equal to or less than in inches, where r 'is the resistivity of the material of which the liner is manufactured in microhm centimeters, r is the mean radius of the liner, and f is the frequency of the applied electric power, and u is the magnetic permeability of the liner material.

9. Molten metal stirring apparatus comprising a ladle for receiving molten metal to be stirred, a plurality of phase coils wound around said ladle along the vertical axis thereof, said coils being axially aligned and spaced one from another, said coils being external of said ladle, a low frequency polyphase supply system supplying electrical power to said coils, said coils progressively varying in electrical phase angle, said polyphase supply system frequency being chosen so that the depth of current penetration d of the molten metal is less than the radius of the molten metal to be stirred, said depth of current penetration d being equal to 1-985\/T/,1Lf, where T is the resistivity of the molten metal in microhm centimeters, a is the magnetic permeability of the molten metal, and f is the frequency.

10. The molten metal stirring apparatus of claim 9 wherein the ratio of the depth of current penetration of the molten metal to the radius of the molten metal varies between the limits of .3 and .9.

11. The molten metal stirring apparatus of claim 9 wherein the ratio of the depth of current penetration of the molten metal to the radius of the molten metal varies between the limits of .5 and .8.

12. Molten metal stirring apparatus comprising a ladle for receiving molten metal to be stirred, a plurality of phase coils wound around said ladle along the vertical axis thereof, said coils being axially aligned and spaced from one another, a low frequency polyphase system supplying electrical power to said coils, said coils progressively varying in electrical angle, said ladle including a metal, non-magnetic outer shell with a refractory liner, the thickness of said shell t being less than d 51' where d is the depth of current penetration of the steel shell, and r is the mean radius of the shell, said phase coils including two sets of coils, each coil in one set of coils being provided with a respective magnetically reversed coil in the second set of coils, associated coils in said first and second set being spaced axially one from another with at least one non-associated phase coil being placed between each pair of associated coils in consistent phase progression, said coils adding up to integral multiples of one cycle of the applied electric power, the distance between associated coils being equal to or greater than twice the sum of the radial distance between the inside of the coil and the molten metal in the ladle, plus one-fifth the depth of current penetration d into the molten metal, the depth of current penetration ca in inches being equal to 1.985\/'r/,uf where 'r is the resistivity of the metal in microhm centimeters, ,u is the magnetic permeability of the molten metal, and f is the frequency of the applied electric power, a non-magnetic liner, said phase coils being wound about said non-magnetic liner to protect said coils from damage, said liner having a thickness If equal to or less than P'p pf in inches, where 1,, is the resistivity of the material of which the liner is manufactured in microhm centimeters, r is the mean radius of the liner, and f is the frequency of the applied electric power, and u is the magnetic permeability of the liner material, said polyphase supply system frequency being chosen so that the depth of current penetration d of the molten metal is less than the radius of the molten metal to be stirred, said depth of current penetration ai being equal to 1.985 /,uf.

References Cited by the Examiner UNITED STATES PATENTS 1,839,802 1/1932 Northrup 13-27 1,943,802 1/1934 Northrup 13-27 2,322,618 6/1943 Demare 1327 2,852,586 9/1958 Steele 13 -27 3,162,710 12/1964 Anderson 263--1l XR 3,235,243 2/1966 Taylor 266-34 FOREIGN PATENTS 836,356 6/1960 Great Britain.

JOHN F. CAMPBELL, Primary Examiner. J. M. ROMANCI-HK, Assistant Examiner. 

1. MOLTEN METAL STIRRING APPARATUS COMPRISING A LADLE FOR RECEIVING MOLTEN METAL TO BE STIRRED, A PLURALITY OF PHASE COILS WOUND AROUND SAID LADLE ALONG THE VERTICAL AXIS THEREOF, SAID COILS BEING AXIALLY ALIGNED AND SPACED ONE FROM ANOTHER, A LOW FREQUENCY POLYPHASE SUPPLY SYSTEM SUPPLYING ELECTRICAL POEWR TO SAID COILS, SAID COILS PROGRESSIVELY VARYING IN ELECTRICAL PHASE ANGLE, SAID COILS BEING CONNECTED TO INCLUDE ONE MAGNETICALLY REVERSED COIL FOR EACH COIL DIRECTLY CONNECTED TO THE POLYPHASE SUPPLY SYSTEM. 